1. Field of the Invention
This invention relates to a plant fault diagnosis system capable of diagnosing fault or failures and applied to a large scale plant such as chemical plants.
2. Description of the Related Art
Hitherto, a diagnosis system for diagnosing faults and failures generated at plants has a function capable of detecting a fault generated in a process of the plant and provided with a predetermined threshold upon the process value, whereby a fact that the process value deviates from the threshold is notified as an alarm to operators.
However, with the above-described type of method, when a fault is generated in a process and the disturbance of the process is propagated, the number of alarms increase. Therefore, it becomes very difficult for the operator to acknowledge the type of the fault generated in the process and the cause thereof. To this end, as means to overcome the above-described type of problem, a method is known in which the number of alarms are reduced by displaying the logical product of a plurality of these alarms which are considered to represent the same information as the representative value of the alarms. Another known method is such that the way to display the critical alarm is distinguished from general alarms.
However, according to the above-described technology, even if any fault is detected in a process, only detected is the disturbance generated in the process. Therefore, the cause for the faults cannot be identified. A method can be considered to identify the cause of a fault by a pattern matching way such that the alarm items expected to be generated in accordance with the causes for faults are previously picked up and the logical product thereof is used. However, with this method, all of the alarms need to be issued in the same form to identify the cause. That is, a problem arises that the cause cannot be detected if this type of alarm cannot be detected in a case where the pattern matching method is applied to the alarm of a poor probability to be detected such as an alarm representing the disturbance degree of the process value is too small and a certain time delay is expected to occur from generation of the fault to detection of it.
A plurality of instruments such as pressure gauges and differential pressure gauges provided for chemical plants or the like are critical instruments for acknowledging the state of the operating condition of the plants. Although these instruments need to always work accurately, they can generate erroneous indications due to an indication level hold and a deviation after they have been used for a long time period. Hitherto, since means to automatically detect such fault of the instruments have not been available, it needs to depend upon the detecting means of human power of the operators. That is, the operator performs a fault detection by way of detecting the erroneous indication of the instruments after they have checked the indicated levels by the instruments from the fault state through the plant, or after they have made a comparison the indicated levels to those of the field instruments.
However, a problem arises that a quick and proper fault detection cannot be performed if such detection depends upon the operator's human power. As a result of this, it is feared that, for example, although the pressure of a plant is at the abnormal level, the erroneous indication shown on the instrument causes the operator to determine that the pressure is in the normal range. It leads to a fact that an early detection of the faults in the plant cannot be performed. On the contrary, it as well feared that although the pressure is in the normal range, the erroneous indication shown on the instrument causes the determination of the abnormal pressure. As a result of this, there is the possibility of performing erroneous operation.
As a means for diagnosing fault in the plant there are off-line fault diagnosing systems in which fault diagnosing is performed by inputting process data of the plant in off-line-input, and on-line fault diagnosing system in which the data is on-line-input. Process data to be input to the types of diagnosing systems are, without exception, measured by the corresponding instruments. In an off-line system, the result of field reading the indication is manually input, while in an on-line system, data is input through signal lines. However, for example, in chemical plants, the terminate end of the detecting portion of the instrument is corroded due to the characteristics of the subject to be treated or clogging can be generated. In the instruments having movable portions, the movable portions can be caught or the like, causing for the normal working to be prevented.
As described above, in a great size plants such as chemical plants, if the fault diagnosing is performed by using the process value of the defected instruments, fault generation can be detected although any fault is not generated, causing for an alarm or the like to be erroneously issued. Such erroneous indication causes for operators to become needless confusion, and is unfavorable on the viewpoint of operating plants.
At the time of performing plant instrumentation, the upper limit and the lower limit of the normal operation range with respect to each of the processes are provided and a device for issuing an alarm when the process value deviates from the above normal operation limits is installed for the purpose of preventing generation of breakdowns due to generation of faults at the time of instrumentation.
When a fault is generated in the instruments of a chemical plant, the local disturbance successively propagates to other portions widely. As a result of this, only a sole failure affects so many process values. It needs to identify the cause of the generation of the fault so as to remove it, but it is also important to maintain each of the disturbed process values within a safety range. The reason for this lies in that entering of the disturbed process value due to the original failure into the unfavorable range can cause another failures. If a multiplicity of process value disturbance are simultaneously generated, it needs to select the disturbance of the process value capable of causing a critical failure from a large number of alarms so as to preferentially act to overcome the critical failure.
Therefore, in the instrumentation of chemical plants, systems were disclosed with which an alarm is issued real timely for notifying the generation of a fault by performing a fault diagnosis with computers. In these alarm systems, a plurality of alarms are issued for each process to which the disturbances are propagated and, in order to select the process value capable of causing a critical failure from each of the process values which have generated disturbances, a classification is employed such as that, for example, the color of the alarm displaying lamp is classified in accordance with the degree of importance factor.
However, the alarm systems of the type as described above confront the following problems.
That is, such systems notify a fact that an abnormal state was generated in the instrumentation of the chemical plant after this state has occurred, but these systems cannot issue alarms previously by detecting a fact that the process value is now deviating from a normal range. Therefore, it arises a problem that an action to overcome the abnormality in a chemical plant is caused to be too much delayed.
On the other hand, a method can be employed in which the upper and lower limits of the process value are provided in a further safety range so that a fault is detected in an early stage. However, there is the probability of issuing an erroneous information depending upon a determination of fault although the state is normal.
Furthermore, the alarms are selected only on the basis that the importance factor of the fault of the chemical plant generated from deviation from a normal range of the process value, but selection of issue of the alarms in accordance with a floating time (marginal time) taken to reach the generation of the fault is not considered. As a result of this, although there is insufficient floating time to generation of a fault, and although the importance factor of such fault is critical, no alarm is not issued, causing a fault.
Furthermore, a conventional plant fault diagnosing system is known in which the physical quantity of each of the portion in a plant is always measured with sensors for the purpose of issuing an alarm when the physical quantity or its change rate is excessively different from the value at the normal operation sate of the plant.
However, in these type of plant fault diagnosing systems confront the following problems.
Since the allowable range for the physical quantity is arranged such that the physical quantity can be present applicable to all of the operation conditions of the plant, the allowable range needs to be arranged large. In such case, a certain long time is taken from generation of a fault in the plant to issue of an alarm, and it becomes a cause for the successive generation of faults in each of the portions in the plant. As a result of this, an excessive long time period is taken to recover the fault.
When an alarm is set for the change rate, such setting can cause alarm issue depending upon a determination of generation of the fault although the state cannot actually be determined that the fault occurred. That is, an alarm can be issued relatively earlier from the start of the physical quantity change due to generation of the fault, however, the alarm is obliged to be issued even if the measured value contains a disturbance such as noise or when the physical quantity changes slightly.
Furthermore, since the number of points at which the physical quantity is measured is large in the plant instrumentation, it takes too much labor for operators to completely and manually determine the allowable range of the fluctuation with respect to the value at the time of normal operation.